In February 1984, the Department of Transfusion Medicine converted from manual to automated platelet collection techniques. During the next 10 years, 26 donors undergoing apheresis procedures on the Fenwal CS-3000 device experienced acute hypersen-sitivity reactions. Sixteen reactions occurred during plateletpheresis and 10 reactions during granulocytapheresis procedures. Using a combination of skin tests, radioallergosorbent tests (RAST), and basophil histamine release assays, we found specific IgE-mediated sensitization to ethylene oxide (EO) (a gas used to sterilize the plastic disposable apheresis kits) in 10 of 16 plateletpheresis donors and 8 of 10 granulocytapheresis donors experiencing reactions, but in none of the 140 nonreacting controls. Donors with documented EO sensitization were permanently deferred from subsequent apheresis donations. The results of these studies were reported to the manufacturer of the CS-3000 apheresis device and to the FDA. As a result of these reports, the manufacturer of the CS-3000 disposable apheresis kits changed their sterilization techniques from predominantly EO exposure to predominantly gamma irradiation. Since this change, there have been only two documented cases of EO hypersensitivity reactions in DTM donors, in August 1995 and March 1998. However, in August 1997, a donor in another blood center had an acute fatal anaphylactic reaction during plateletpheresis and was found on postmortem testing to have high titer IgE anti-EO. The donor also had a strong history of asthma. This first report of a lethal allergic reaction to EO in an apheresis donor has reopened the question of prospective screening of all apheresis donors for EO sensitization. We have estimated that as many as 1.0 percent of all repeat apheresis donors may become sensitized to EO, although only a fraction of those who are sensitized will have clinically evident allergic reactions. To document the current EO sensitization rate among donors and to compare this rate with individuals who have occupational exposure to EO, we have established a collaborative effort with CBER/FDA. Screening of approximately 500 healthy repeat apheresis donors using both an established RAST test and an experimental Energy Information Administration test for IgE anti-EO will be performed. A cohort of serum samples stored at the Centers for Disease Control and Prevention and derived from individuals with allergic reactions presumed to be due to EO will also be tested, as well as a large sample of samples derived from the National Health and Nutrition Examination Survey study.